The normal value of free thyroxine is 11.5~22.7 pmol/L (chemiluminescence method), but the reference range may be different in different hospitals. Free thyroxine is a commonly used indicator of thyroid function, and its measurement is not affected by plasma thyroxine-binding globulin, which can directly reflect the functional status of the thyroid gland. Decreased free thyroxine, less than 11.5 pmol/L, is mainly seen in hypothyroidism, mucous edema, advanced Hashimoto’s thyroiditis and other diseases. Increased free thyroxine, greater than 22.7 pmol/L, is diagnosed considering conditions such as hyperthyroidism, hyperthyroid crisis, and multinodular goiter. The diagnosis of a thyroid problem relies on the results of thyrotropin, free triiodothyronine, and thyroid binding globulin in addition to free thyroxine. If a patient has an abnormal free thyroxine value, it is recommended that he or she seek prompt medical attention and standardized treatment under the guidance of a doctor.